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NASA’s Mars rover Perseverance may have found a major clue central to its Mars mission: geological evidence that could suggest life existed on the red planet billions of years ago.

The robotic explorer came across a vein-filled red rock on July 18 that appears to be dotted with leopard spots. The stains may indicate that ancient chemical reactions occurring in the rock once supported microbial organisms.

“These spots are a big surprise,” David Flannery, a member of NASA’s Perseverance science team and an astrobiologist at Queensland University of Technology in Australia, said in a statement. “On Earth, these types of features in rocks are often associated with the fossilized record of microbes living below the surface.”

The research is still preliminary, and NASA scientists have not yet confirmed how the rock was formed, which would require studying it on Earth. But the arrowhead-shaped specimen could help the Perseverance team figure out whether Mars was once a habitable planet.

“We are absolutely thrilled to have this sample in the bag!” said Bryony Horgan, co-investigator of the Perseverance rover mission and professor of planetary sciences at Purdue University in West Lafayette, Indiana, in an email.

“This rock is exactly the type of sample we came to Mars to find, and we can’t wait to bring it back to our labs here on Earth,” she said. “This is exactly the type of potential microbial biosignature that was envisioned when NASA designed the Mars 2020 mission, and we used every instrument in our payload to find and understand this rock.”

The rock, named Cheyawa Falls after one of the Grand Canyon’s waterfalls, intrigues scientists for a number of reasons.

The white veins of calcium sulfate are clear evidence that water – crucial to life – once flowed through the rock. The rover used its Habitable Raman & Luminescence for Organics & Chemicals, or SHERLOC, instrument to identify organic carbon-based molecules in rocks.

And the irregularly shaped leopard spots tested by the rover’s PIXL instrument, short for Planetary Instrument for X-ray Lithochemistry, found iron and phosphate in the features, Morgan Cable, a researcher on the rover’s team, said in a video shared by the Jet Propulsion Laboratory of NASA in Pasadena, California.

“We’ve never seen these three things together on Mars before,” Cable said.

The team also noticed the potential presence of hematite between the white streaks of calcium sulfate in the rock. Hematite is one of the minerals responsible for Mars’ characteristic red hue.

The leopard sighting may have occurred when chemical reactions with hematite turned the rock from red to white, which could release iron and phosphate and potentially cause black rings to form. Such reactions can also provide a source of energy for microbes.

“Cheyawa Falls is the most puzzling, complex and potentially important rock yet studied by Perseverance,” Ken Farley, Perseverance project scientist and professor of geochemistry at the Cal Institute of Technology in Pasadena, said in a statement.

The team also found millimeter-scale olivine crystals in the same rock. Olivine, previously found in another part of Perseverance Crater, is a mineral that forms from magma. According to the team, the olivine present in the Cheyawa Falls rock may be related to rocks formed elsewhere in the valley.

The rover team grapples with a host of questions as they study the rock and try to determine what processes could have formed it.

Cheyawa Falls may have started as a mixture of deposited mud and organic compounds that eventually cemented to become rock. Later, water may have seeped through cracks in the rock, depositing minerals to create calcium sulfate veins and leopard spots.

But it’s also possible that olivine and sulfate became part of the rock due to Mars’ scorching hot temperatures, causing a non-biological chemical reaction that created the leopard spots.

After landing on Mars, Perseverance traversed the Jezero crater and explored the delta of an ancient river in search of microfossils from a past life. The rover has been collecting samples along the way that could be returned to Earth by future missions.

Most recently, Perseverance explored the northern end of the Neretva Valley, an ancient river valley that once supplied water to Jezero Crater more than 3 billion years ago, and there spotted Cheyava Falls. The rover landed in the crater to survey the site of the ancient lake in February 2021.

Geologists on the rover team have been eager for Perseverance to study rocks that were created or modified by water on Mars in the past, which is why Cheyawa Falls intrigued them.

“We designed the route for Perseverance to ensure that it goes to areas with the potential for interesting science samples,” said Nicola Fox, associate administrator for NASA’s Science Mission Directorate, in a statement. “This trip across the Neretva River bed paid off as we discovered something we’ve never seen before that will give our scientists so much to study.”

In April, NASA said the original complex, multi-mission design for the program to return Perseverance samples to Earth, called Mars Sample Return, was no longer feasible in its current architecture due to budget cuts and a delayed return date.

The agency invited NASA centers and industry to develop a new plan that combines innovation with lessons learned from proven technologies. NASA management’s hope is to return samples to Earth by 2030 with less complexity, cost and risk than originally planned, and the agency expects to have answers on how best to return samples from Mars by the fall, said NASA Administrator Bill Nelson during a press conference in April.

Meanwhile, Perseverance continues its crucial exploration work on Mars and will soon begin an ascent of the Jezero crater rim.

“This discovery comes at such a critical time as NASA reviews the best way to return these samples from Mars through Mars Sample Return,” Horgan said. “This shows how important and unique our sample set (is) and how much we can learn about the beginnings of life on Earth-like planets. It also feels very fitting that Jezero has one last surprise for us before we leave the ancient river and lake sediments at the bottom of the crater and begin climbing the rim.”

The Perseverance team says returning the samples is the only way to know if life once existed on Mars.

“We imaged this rock with lasers and X-rays and photographed it literally day and night from almost every angle you can imagine,” Farley said. “Scientifically, Perseverance has nothing more to give. To fully understand what really happened in the valley of this Martian river in the Jezero crater billions of years ago, we would like to bring the Cheyava Falls sample back to Earth so that it can be studied with the powerful instruments available in laboratories.